A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Fuel cells are a clean and efficient power source, and may replace traditional power sources such as the internal combustion engine in both stationary and automotive power applications.
In a polymer electrolyte membrane (PEM) fuel cell, the electrolyte is a solid polymer membrane which is electronically insulating but ionically-conducting. Proton-conducting membranes based on perfluorosulphonic acid materials are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.
The principle component of a polymer electrolyte fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer membrane. On either side of the membrane there is an electrocatalyst layer, containing an electrocatalyst. The anode electrocatalyst catalyses the electrochemical oxidation of the fuel, and the cathode electrocatalyst catalyses the electrochemical reduction of oxygen. Finally, adjacent to each electrocatalyst layer there is a gas diffusion substrate. The gas diffusion substrate must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the substrate must be porous and electrically conducting.
The MEA can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion substrate to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of a membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the membrane to form a catalyst coated membrane (CCM). Subsequently, gas diffusion substrates are applied to both faces of the catalyst coated membrane. Finally, an MEA can be formed from a membrane coated on one side with an electrocatalyst layer, a gas diffusion substrate adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the membrane.
WO 00/10216 discloses an MEA made from a CCM wherein a sub-gasket is positioned between the catalyst layer and the gas diffusion substrate. EP 1 403 949 discloses an MEA made from a CCM wherein a protective film layer is attached to the membrane and overlaps both the passive area and the active (catalysed) area of the membrane. EP 1 403 949 states that the CCM with the protective film layer offers improved mechanical stability and improved protection against membrane damage. The Examples of EP 1 403 949 demonstrate that the MEAs are durable for 300 hours. The present inventors have sought to further improve the durability of MEAs.